The objectives are to characterize the effect of age on the physiology of cardiac muscle contraction and on the response of cardiac muscle to inotropic interventions. It had been demonstrated the previous year that contraction duration was prolonged in aged myocardium and that a prolonged active state resulting from delyed calcium removal from the contractile element may be responsible for this phenomenon. It had also been shown that age impairs the ability of cardiac muscle to mount an inotropic response to catecholamines. The present year's efforts were directed towards: (1) Examining the effect of age on the ability of the cardiac cell membrane to respond appropriately to an inotropic intervention. This was accomplished by examining the responsiveness to ouabain, a digitalis preparation. The physiology of the response was assessed by comparing the inotropic response to paired pacing with that to ouabain. The biochemical aspects of the response was determined by measuring the degree of ouabain binding to a membrane preparation and by measuring the degree of inhibition of Na-K ATPase by ouabain, (2) Isolating the subcellular fraction thought to be responsible for calcium uptake in order to eventually determine if the age-related increase in contraction duration resulted from changes in this mechanism.